The chart below shows the relative humidity and temperature and how they interplay to create lethal conditions such as heat stroke and death. See the second chart below which is in Celsius for recommendations to protect yourself.
National Weather Service health guidelines for extreme heat: https://www.weather.gov/safety/heat
Below is a similar chart in Celsius temperature readings. Scroll to the second page. SEE RECOMMENDED ACTIONS LISTED BY WET BULB TEMPERATURE.
[Note: I am reposting this information because we are experiencing a very hot period in the USA and across the planet. See the NY Times listing of temperatures predicted for various cities in the USA]
The National Collaborating Centre on Environmental Health in Canada prepared this very helpful document on health checks during extreme heat events. Download and keep it with you and your family or coworkers if you are in extreme heat events. Also, see this related post explaining wet bulb science.
UPDATED May 15, 2023
Washington Post article: What Extreme Heat Does to the Human Body
Susan Feathers 